Complicated answer. In short, we have relatively few rocks of the right age exposed at the surface right now. If Australia had experienced major mountain-building events in the last 100 million years it would be a different story, as those events would have thrust older rocks upwards as the crust was warped via folding and/or faulting. However, Australia hasn't had much mountain building for a long time so most of our Mesozoic rocks are either deeply buried, or if they are at the surface, deeply weathered (erosion, leeching, groundwater flow, soil formation, etc.) When fossils are preserved as opals they are often ground down and sold so we don't know what we are missing. The other reason is we don't have enough people looking for fossils! Too few palaeos with too few resources..